Display apparatus

ABSTRACT

The present invention provides a bright and high-resolution display apparatus having a dynamic range exceeding the number of gray-scale voltage (or current) outputs, which a signal driver is capable of generating. In accordance with the present invention, a select period, in which a group of pixels on each row is driven, is divided into a plurality of sub-periods. The signal driver supplies a voltage output varying from sub-period to sub-period to selected pixels through a signal electrode. The pixel is capable of expressing various values of a gray scale, the size of which is at least approximately equal to (the number of gray-scale voltage outputs, which the signal driver is capable of generating)×(the number of sub-periods). By changing the ratio of the length of a sub-period to the length of another sub-period or the range of the driving voltage (or current), the dynamic range of the display can be further increased.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a flat display apparatus such asan FED (Field Emission Display) unit using display elements comprisingtypically electron emission devices laid out to form a matrix and piecesof fluorescent material each emitting light due to electrons emitted bythe electron emission devices.

[0002] As the electron emission devices, a MIM (Metal-Insulator-Metal)type electron source is used. The MIM electron source has a structurecomprising three thin film layers, which serve as an upper electrode, aninsulator and a lower electrode respectively. The display apparatusadopts an FED driving technique connecting the upper electrode to acolumn electrode (or a signal electrode) and the lower electrode to arow electrode (or a scanning electrode). A typical FED driving techniqueis disclosed in Japanese Patent Laid-open No. 2001-83907. In accordancewith this reference, a scanning electrode is associated with a group ofpixel rows and the pixel group is driven sequentially one row afteranother.

[0003] A second prior art is disclosed in Japanese Patent Laid-open No.2002-341365. In accordance with this reference, a scanning electrodedriven sequentially is associated with a group of pixels on pair rows toform a double-matrix electron pattern used in a liquid crystal drivingcircuit. The second-prior art can be applied also to the FED unit.

SUMMARY OF THE INVENTION

[0004] In accordance with the first prior art described above, thepixels are driven sequentially one row after another. Thus, in the caseof a high-resolution panel, a select period of a row is short. As aresult, this technique is prone to a lack of a driving timing margin. Inaddition, since a light emission period is also short, there is raised aproblem of a difficulty to obtain a high intensity of light.

[0005] In addition, in accordance with the first prior art, by properlychanging the magnitude of a voltage applied to the signal electrode to alevel adjusted to a picture signal, a picture with various gray-scalelevels can be generated. The voltage applied to the signal voltage is avoltage for driving the electron emission devices. Thus, in order toproduce a TV picture having a high quality, it is desirable to set thenumber of bits for digital image data at a value in the range of 8 to12. The number of bits is the base of the driving voltage cited above.That is to say, the number of bits is the input-bit count of a D/A(Digital to Analog) converter for converting the digital image data intoan analog driving voltage. In general, however, the input-bit count of aD/A converter serving as a driver for applying the driving voltage tothe signal electrode is a value in the range of 6 to 8. Thus, if anordinary D/A converter is employed, the size of the gray scale is avalue in the range of 64 to 256. For this reason, it is desirable tofurther improve the gray-scale performance of the FED unit. Thegray-scale performance is referred to as a dynamic range.

[0006] Let the second conventional technology be applied to, forexample, the FED unit described as a part of the first prior art. Inthis case, pixels on two rows are driven at the same time. Thus, theselect period of a pixel group comprising two rows is twice thecorresponding period according to the first prior art. As a result, adriving timing margin can be assured with ease. In addition, since thelight emission period is also increased, there is offered an advantagethat it is easy to produce a high light intensity. Like the first priorart described above, however, also in the case of the second technique,the dynamic range of the light emission is limited by the input-bitcount of the D/A converter serving as the signal driver. Thus, it isimpossible to display a picture with a gray-scale size greater than thegray-scale size, which is determined by the input-bit count of the D/Aconverter.

[0007] It is an object of the present invention to address the problemsdescribed above to display a bright picture with a high resolution byimproving the gray-scale performance. To put it concretely, it is anobject of the present invention to improve the gray-scale performance byproviding a capability of displaying a picture with a gray-scale sizegreater than the gray-scale size determined by the input-bit count ofthe D/A converter, which serves as the signal driver.

[0008] In addition, it is a second object of the present invention toimprove the brightness of picture with good gray-scale performance.

[0009] In order to achieve the first object described above, the presentinvention is characterized in that, during a select period to select atleast one row of a plurality of display devices (electron emissiondevices), which are laid out to form a matrix, at least two drivingvoltages with levels different from each other are applied to theselected display devices. The select period is a period during which aselect voltage is being applied to the scanning electrode. To put it indetail, in accordance with the present invention, the select period isdivided into a plurality of sub-periods and, during each of thesub-periods, a driving voltage with a level different from levels of thedriving voltages applied during the other sub-periods is applied to theselected electron emission devices.

[0010] In accordance with the configuration of the present invention, apixel corresponding to a driven electron emission device is capable ofrealizing a display with the number of gray-scale levels at least equalto about (the number of gray-scale voltage levels that can be output bythe signal driver)×(the number of sub-periods in the select period).Assume for example that the 8-bit D/A converter serving as the signaldriver has an input-bit count of 8. In this case, the number ofgray-scale voltage levels that can be output by the signal driver is256. Thus, if the select period is divided into two sub-periods, a pixelhas an ability to display 512 (=2×256) gray-scale levels. That is tosay, in accordance with the present invention, it is possible to realizea multiple gray-scale display with many gray-scale levels exceeding themaximum gray-scale levels, which is determined by the input-bit count ofthe D/A converter serving as the signal driver.

[0011] In addition, in order to achieve the second object of the presentinvention, besides the present invention's characterization describedabove, the present invention is further characterized in that aplurality of rows of electron emission devices is driven at the sametime. For example, as a plurality of rows to be driven at the same time,two adjacent rows may be selected. In this case, one of the two rowsselected at the same time may be selected again during another selectperiod. In this way, the length of a select period for each row, thatis, a select period for pixels on the row, can be increased. Thus, theintensity of light can be increased with ease and the signal driver canbe relieved from a requirement of a high operation speed due to thedivision of the select period.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a block diagram showing a first embodiment of a pixellayout and electrode wiring of a display apparatus provided by thepresent invention;

[0013]FIG. 2 is a diagram showing the waveforms of driving signals inthe display apparatus provided by the present invention;

[0014]FIG. 3 is a diagram showing the waveform of a driving signalgenerated by a signal driver to generate a typical gray scale display inaccordance with the present invention;

[0015]FIG. 4 is a block diagram showing an embodiment implementing thedisplay apparatus provided by the present invention;

[0016]FIG. 5 is a block diagram showing another embodiment implementingthe display apparatus provided by the present invention;

[0017]FIG. 6 is a block diagram showing an embodiment implementing aTa/Tb signal converter employed in the display apparatus shown in FIG.5;

[0018]FIG. 7 shows a truth table showing typical operations of the Ta/Tbsignal converter employed in the other embodiment shown in FIG. 5;

[0019]FIG. 8 is a block diagram showing a second embodiment of a pixellayout and electrode wiring of the display apparatus provided by thepresent invention;

[0020]FIG. 9 is a diagram showing the waveforms of driving signals forthe second embodiment shown in FIG. 8;

[0021]FIG. 10 is a diagram showing an electrode pattern for the secondembodiment shown in FIG. 8;

[0022]FIG. 11 is a diagram showing a perspective view of spacers and arear substrate, which are applied to the present invention;

[0023]FIG. 12 is a block diagram showing a third embodiment of a pixellayout and electrode wiring of the display apparatus provided by thepresent invention;

[0024]FIG. 13 is a block diagram showing a fourth embodiment of a pixellayout and electrode wiring of the display apparatus provided by thepresent invention; and

[0025]FIG. 14 is a diagram showing the waveforms of driving signals forthe fourth embodiment shown in FIG. 13.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0026] Embodiments of the present invention will be explained in detailwith reference to the diagrams. FIG. 1 is a block diagram showing afirst embodiment of a pixel layout and electrode wiring of a displayapparatus provided by the present invention. The display apparatusimplemented by the embodiment comprises a plurality of pixels P11, P12and so on, a plurality of scanning electrodes S1, S2 and so on, aplurality of signal electrodes DO1, DE1, DO2, DE2 and so on, a scanningdriver 201 and a signal driver 301. The pixels P11, P12 and so on arelaid out to form a matrix. The scanning electrodes S1, S2 and so on areeach extended in the horizontal direction of the screen, each forming arow of the matrix. The signal electrodes DO1, DE1, DO2, DE2 and so onare each extended in the vertical direction of the screen, each forminga column of the matrix. The scanning driver 201 is a driver for applyinga select voltage to a desired row in order to select the row. The signaldriver 301 is a driver for applying a driving voltage to a signalelectrode in order to drive pixels on the signal electrode. The pixelsPij are each located at an intersecting point of one of the scanningelectrodes Si and one of the signal electrodes DOj/DEj. In addition, thepixel Pij is connected to the scanning electrode Si and the signalelectrode DOj/DEj. Thus, a select voltage and a driving voltage aresupplied to the pixel Pij by the scanning electrode Si and the signalelectrode DOj/DEj respectively. FIG. 1 shows the pixel layout's enlargedpartial model comprising (4×4) pixels in a matrix consisting of 1,920columns of pixels and 1,080 rows of pixels. It is needless to say thatthe pixel matrix is not limited to the matrix consisting of 1,920columns of pixels and 1,080 rows of pixels.

[0027] A pixel on any specific one of the odd-numbered scanningelectrodes S1, S3 and so on is connected to one of the odd-numberedsignal electrodes DO1, DO2 and so on that crosses the specificodd-numbered scanning electrode Si. Likewise, a pixel on any specificone of the even-numbered scanning electrodes S2, S4 and so on isconnected to one of the even-numbered signal electrodes DE1, DE2 and soon that crosses the specific even-numbered scanning electrode Si. Bytaking the FED unit employing MIM electron emission devices as describedin patent reference 1 as an example, the operation of the displayapparatus comprising the pixels P11, P12 and so on as the MIM electronemission devices as shown in FIG. 1 is explained by referring towaveforms shown in FIG. 2. In the following description, an MIM electronemission device is referred to simply as an MIM.

[0028] The FED unit comprises a rear substrate and a front substrate,which are placed in such a manner that the rear substrate and the frontsubstrate face each other. The pixels P11, P12 and so on each serving asan electron emission device, the scanning electrodes S1, S2 and so on,the signal electrodes DO1, DE1, DO2, DE2 and so on, the scanning driver201 and the signal driver 301 are created on the rear substrate to formthe pattern and the connection wiring, which are shown in FIG. 1. On theother hand, pieces of fluorescent material are each created on thefront-surface substrate at a location corresponding to one of theelectron emission devices forming the matrix on the rear substrate. Thepieces of fluorescent material each comprise an R fluorescent materialemitting red light, a G fluorescent material emitting green light and aB fluorescent material emitting blue light.

[0029] A MIM has an upper electrode, a lower electrode and an insulationfilm between the electrodes. When a strong electric field is built up inthe insulation film due to a driving voltage applied between the upperelectrode and the lower electrode, electrons are injected from the lowerelectrode to the upper electrodes by way of a conduction band in theinsulation film, becoming hot electrons. Some of the hot electronshaving much energy surmount the upper electrode, being emitted to avacuum. The emitted electrons are accelerated by applying a high voltageof the order of 3 to 6 kV to an acceleration electrode, which is locatedat a position in close proximity to the pieces of fluorescent materialon the front substrate. The emitted electrons then hit the pieces offluorescent material each provided at a location corresponding to one ofthe electron emission devices. The incident electrons excite each pieceof fluorescent material, causing the fluorescent material to emit lightwith a color according to the emission characteristic. The lowerelectrode is connected to one of the scanning electrodes S1, S2 and soon, which apply a select voltage generated by the scanning driver 201.On the other hand, the upper electrode is connected to one of the signalelectrodes DO, DE1 and so on, which apply a driving voltage generated bythe signal driver 301.

[0030] The operation of the block diagram shown in FIG. 1 is explainedin more detail by referring to FIG. 2. In the period t1 to t3, thescanning driver 201 applies a select electric potential V_(s1) to thescanning electrode S1 connected to the lower electrode of the MIM of thepixel P11. At the same time, the signal driver 301 applies an electricpotential V_(D1) to the signal electrode DO1 connected to the upperelectrode. In this state, a voltage (V_(D1)-V_(s1)) is applied to theinsulation film of the MIM. The MIM of the pixel P11 emits electrons,the number of which is proportional to the voltage. The electrons arethen radiated to a fluorescent material corresponding to the pixel P11,causing the fluorescent material to emit light. The pixel P12 is alsoconnected to the same scanning electrode S1 but connected to the signalelectrode DO2, which receives an electric potential V_(DO) from thesignal driver 301 during the period t1 to t3. Thus, a voltage(V_(DO)-V_(s1)) is applied to the insulation film in the MIM of thepixel P12. If the electric potential V_(DO) is set at such a value thatthe voltage does not exceed a threshold value of the MIM, the MIM doesnot operate so that a fluorescent material corresponding to the pixelP12 does not emit light. The threshold value is the lower limit of theapplied voltage required for operating the MIM.

[0031] In a period after the time t3, a deselect electric potentialV_(s0) is applied to the scanning electrode Si so that the appliedvoltage does not exceed the threshold value without regard to which ofthe electric potentials V_(DO) and V_(D1) is applied to the signalelectrode DO1. As a result, no fluorescent material corresponding to anMIM on the deselected row is emitting light because the MIM is notoperating in spite of the fact that the driving electric potentialV_(D1) is applied to the MIM.

[0032] As described above, among the MIMs laid out to form a matrix,only MIMs pertaining to a selected row are each selected as an operatingMIM, which is a MIM put in a state of being capable of operating. Theselected row is a row, to which the scanning driver 201 applies theselect electric potential V_(s1). To put it concretely, the selected rowis either the scanning electrode S1 or the scanning electrode S2 orboth. If a driving electric potential is further applied to a selectedMIM, the MIM will emit electrons, the number of which is dependent onthe driving electric potential.

[0033] Likewise, the select electric potential V_(s1) is applied to thescanning electrode S2 serving as the second row during the period t2 tot4, which is a select period lagging behind the select period of thescanning electrode S1 serving as the first row by half the selectperiod. The period t2 to t3 becomes a period in which the scanningelectrodes S1 and S2 are selected at the same time. However, the pixelspertaining to the scanning electrode S1 are connected to theodd-numbered signal electrodes DO1, DO2 and so on whereas the pixelspertaining to the scanning electrode S2 are connected to theeven-numbered signal electrodes DE1, DE2 so that a display by the pixelspertaining to the scanning electrode S1 and a display by the pixelspertaining to the scanning electrode S2 can be obtained independently ofeach other. Thereafter, subsequent select operations are carried outsequentially with each select operation delayed from the immediatelypreceding select operation by half the select period. As a result, anyarbitrary picture display can be obtained by independently emittinglight during every select period of each of the rows.

[0034] Next, the configuration of the gray scale according to thepresent invention is explained. As the signal driver 301, typically, adriver including an embedded so-called 8-bit D/A conversion function isused. Such a driver is capable of outputting a voltage for 256gray-scale levels. It is thus obvious that a display with 256 gray-scalelevels can be obtained. In accordance with the present invention, eachMIM's select period determined by the output period of a select voltagegenerated by the scanning driver 201 is divided into two sub-periods,namely, a first sub-period and a second sub-period. The output period isthe duration of the select electric potential V_(s1). During each of thefirst sub-period and the second sub-period, any one of 256 gray-scaledriving voltages independent from each other can be applied. Thus, thesize of the gray scale that can be displayed is substantially twice thenumber of voltage outputs generated by the signal driver 301. As aresult, a display with a gray-scale size of 511 can be realized.

[0035] If at least first and second driving voltages having levelsindependent from each other are applied to MIMs of a certain row duringthe select period of the row, that is, if first and second drivingvoltages with the levels thereof adjustable independently of each otherare applied to the MIMs, what is visible to the human eyes is a resultof addition of emitted light represented by the first driving voltageand emitted light represented by the second driving voltage. Thus, evenif the gray scale levels represented by each of the first and seconddriving voltages is k, that is, even if the quantity of the emittedlight is 0, 1, 2, - - - , or (k−1), a picture can be expressed in termsof (2k−1) gray scale levels, that is, the picture can be displayed as aresult of emission with light quantities of 0, 1, 2, - - - , (k−1), k,(k+1), - - - and (2k−2). In this way, by dividing the select period intosub-periods and applying driving voltages with independent levels toeach of the sub-periods, the gray scale levels can be increased toapproximately the product of the number of such driving voltages and thenumber of sub-periods. It is to be noted that the figure shows a case inwhich the pixels P32 and P42 emit electrons only during the firstsub-period Ta of each of their select periods.

[0036] In addition, the lengths of the first sub-period Ta and thesecond sub-period Tb do not have to be equal to each other. Instead, theratio of the length of the first sub-period Ta to the length of thesecond sub-period Tb is changed so that the dynamic range of the lightemission can be further improved without increasing the number ofsub-periods. In this case, the dynamic range of the light emission isdefined as a ratio of a minimum intensity to a maximum intensity wherethe minimum intensity means a lowest gray-scale level emitted light nextto no emitted light. For example, if the ratio of the length of thefirst-half sub-period Ta to the length of the second-half sub-period Tbis 1:2, the dynamic range can be tripled. In this case, an intensitydifference per gray-scale granularity close to the maximum intensityincreases to a value twice that in a low-intensity portion. Since thedisplay intensity is high, however, there is no problem.

[0037]FIG. 3 is a diagram showing the waveform of a driving signalgenerated by the signal driver 301 and typical allocation of gray-scalegranularities. During a long second sub-period Tb, the voltage level isset at 0. During a short first sub-period Ta, on the other hand, voltagelevels of 0 to 255 are output to generate emitted light of 255gray-scale levels. Pieces of emitted light of the gray-scale range 255to 510 are always generated at a voltage level of 255 in the first-halfsub-period Ta and at voltage levels of 0 to 255 in the second-halfsub-period Tb to provide emitted light of the gray-scale range 255 to510.

[0038] In a low-intensity portion or a dark picture area, the drivingvoltage applied in the longer sub-period (or the second-half sub-periodTa) is set at 0 and the driving voltage applied in the shortersub-period (or the first-half sub-period Tb) is varied so that adifference in intensity between adjacent gray-scale values can be madesmall. Thus, a fine gray-scale display can be obtained. In ahigh-intensity portion or a bright picture area, the driving voltageapplied in the shorter sub-period (or the first sub-period Tb) is set ata maximum value and the driving voltage applied in the longer sub-period(or the second sub-period Ta) is varied so that a difference inintensity between adjacent gray-scale values cane be made large. Sincethe intensities themselves are high, however, the rate of changes inintensity is relatively small so that there is almost no problem in thesense of sight. That is to say, in accordance with the presentinvention, either the first sub-period Ta or the second sub-period Tb isselected in dependence on the intensity of the picture for use incontrol of the gray scale. In addition, there is also offered anadvantage that the monotonous rising characteristic of the level of theemitted light is sustained in spite of the fact that the difference inintensity per step changes from a value on one side of a boundary toanother value on the other side of a boundary, which is the value of the255th gray scale level. The change in intensity difference per step canbe corrected by means of a gray-scale correction circuit using an LUT(Look Up Table). An example of the gray-scale correction circuit is theso-called gamma correction circuit.

[0039] The figure does not show the fact that, by widening the range ofthe driving voltage (or the driving current) applied during the secondsub-period Tb to exceed the range of the driving voltage (or the drivingcurrent) applied during the first sub-period Ta, the difference inintensity per step can also be changed. In addition, by raising the highvoltage applied to a fluorescent material in order to accelerate emittedelectrons during the sub-period Tb, the same effects can also beexhibited even if the duration of the sub-period Ta is equal to theduration of the sub-period Tb and the range of the driving voltage (orthe driving current) applied during the sub-period Tb is about the sameas the range of the driving voltage (or the driving current) appliedduring the sub-period Ta.

[0040] It is to be noted that, even if the duration of the firstsub-period Ta is set at a length equal to the duration of the secondsub-period Tb, due to distortions of the waveform of the driving voltageand other causes, the brightness can be more than doubled byadditionally emitting light during the second sub-period Tb continuouslyfollowing the first sub-period Ta in comparison with a case in whichlight is emitted only during the first sub-period Ta. This difference inintensity per step can be corrected if the gray-scale correction circuitdescribed above is employed.

[0041] By the way, in order to avoid cross-talks and to obtain a stabledisplay gray scale, after the scanning electrode S2 transits to theselect electric potential V_(s1) to enter a select state at the time t2in the driving voltages' waveforms shown in FIG. 2, the signal electrodeDE1 transits to the electric potential V_(D1), lagging behind thetransition of scanning electrode S2 to the select electric potentialV_(s1). In addition, before the scanning electrode S2 starts atransition to the deselect electric potential V_(s0) at the time t3, thesignal electrode DE1 completes a transition to the electric potentialV_(DO). That is to say, by setting the rising edge of the drivingvoltage generated by the signal driver 301 at a time lagging behind eachof the timings t1 , t2 and so on of the starts of the select voltagesoutput by the scanning driver 201 and by setting the falling edge of thedriving voltage generated by the signal driver 301 at a time leadingahead of each of the timings t3, t4 and so on of the starts of theselect voltages, it is possible to offer an advantage that, as a drivingsystem, extra timings no longer need to be set. In this case, the secondhalf of the select period of a row becomes the long second sub-period Tband the duration of the first sub-period Ta can be set at a valueobtained by subtracting a period from duration of the second sub-periodTb. The period subtracted from the duration of the second sub-period Tbcorresponds to a delay of the waveform of the select voltage generatedby the scanning driver 201 and the distortions of the waveform.

[0042] In this embodiment, the light-emission period is divided into twosub-periods, namely, the first sub-period Ta and the second sub-periodTb. However, it is needless to say that, by dividing the light-emissionperiod into three or more sub-periods as described earlier, the dynamicrange of the display can be further increased.

[0043]FIG. 4 is a block diagram showing an embodiment implementing thedisplay apparatus provided by the present invention. Examples of thedisplay apparatus are the monitor provided for a personal computer and aTV receiver. To put it more concretely, the block diagram shows thesignal driver 301 shown in FIG. 1 and a typical signal-processing systemfor-generating a signal applied to the signal driver 301. The signaldriver 301 shown in FIG. 1 includes a signal driver 320 for driving agroup of odd-numbered signal electrodes DO1, DO2 and so on as well as asignal driver 330 for driving a group of even-numbered signal electrodesDE1, DE2 and so on. The signal drivers 320 and 330 have the sameconfiguration including a data distribution circuit 321 for distributingan input signal to columns of the matrix, latch circuits 322 each usedfor latching a distributed signal and D/A conversion circuits 323 eachused for converting a digital signal stored in one of the latch circuits322, which is associated with the D/A conversion circuit 323, into apredetermined analog voltage. The operation of the signal-processingsystem is explained as follows.

[0044] The display apparatus is capable of inputting or receiving bothan analog video signal and a digital video signal. An input analog videosignal is converted into a digital signal by an A/D (Analog to Digital)converter 311. On the other hand, an input digital video signal isdecoded by using a reception interface (RX) 312, which includes adigital decoder. Signals output by the A/D converter 311 and thereception interface 312 are supplied to a switch 313. The switch 313selects one of the signals and supplies the selected signal to agray-scale correction circuit 314, which functions as a driving signalgenerator. The signal selected by the switch 313 is a digital videosignal. Typically including an LUT (Look Up Table), the gray-scalecorrection circuit 314 carries out a gray-scale correction process suchas a gamma correction process to determine the display apparatus'gray-scale value corresponding to the digital video signal.

[0045] The gray-scale correction circuit 314 has a function fortransforming the bit count of the digital video signal received from theswitch 313 into two driving signals. If the D/A conversion circuit 323embedded in the signal drivers 320 and 330 has an input bit count of 8,for example, the gray-scale correction circuit 314 provided by thisembodiment transforms the bit count of the digital video signal into16-bit signals. That is to say, the gray-scale correction circuit 314has a function to transform the bit count of the digital video signalinput thereto into output signals each having a bit count greater thanthe bit count of the digital video signal. The 16-bit signal obtained asa result of the transformation is divided into a first 8-bit drivingsignal and a second 8-bit driving signal. The first 8-bit driving signalserves as the base of a driving voltage applied to the signal electrodein the first sub-period Ta. On the other hand, the second 8-bit drivingsignal serves as the base of a driving voltage applied to the signalelectrode in the second sub-period Ta. In the case of a gray-scale valuelower than an intermediate boundary gray-scale value of 255, the firstdriving signal has a value representing the input video signal while allthe bits of the second driving signal are set to 0. In the case of agray-scale level higher than the intermediate boundary gray-scale valueof 255, on the other hand, the second driving signal has a valuerepresenting the input video signal while all the bits of the firstdriving signal are set to 1 to represent a value of 255.

[0046] By carrying out the operation of the gray-scale correctioncircuit 314 as described above, it is possible to generate the first andsecond driving signals to be applied to the first and second sub-periodsTa and Tb respectively as explained above by referring to FIG. 3. Thefirst driving signal for the first sub-period Ta is output from alower-side terminal of the gray-scale correction circuit 314, beingsupplied to a left-side terminal of a switch 316 and a right-sideterminal of a switch 317. The switches 316 and 317 each function as achangeover switch. On the other hand, the second driving signal for thesecond sub-period Tb is output from an upper-side terminal of thegray-scale correction circuit 314, being supplied to a line memory 315.The line memory 315 delays the second driving signal by a period of timeequal to the first sub-period Ta before supplying the second drivingsignal to a right-side terminal of the switch 316 and a left-sideterminal of the switch 317.

[0047] The video signal selected by the switch 313 is also supplied to acharacteristic extraction circuit 319 for extracting the video signal'scharacteristics such as a white peak level, an average intensity leveland a brightness-classified histogram and supplying results ofextraction to a Tb/Ta control circuit 318. The Tb/Ta control circuit 318carries out an optimum picture-drawing operation by controllingparameters such as a ratio of the length of the sub-period Ta to thelength the sub-period Tb on the basis of the extraction results receivedfrom the characteristic extraction circuit 319. In order to display animage dominated by a dark picture, for example, control is executed toshorten the first sub-period Ta. In order to display an image dominatedby a bright picture, on the other hand, control is executed to lengthenthe first sub-period Ta and adjust the lengths of both the firstsub-period Ta and the second sub-period Tb so as to make the first-halfsub-period Ta substantially equal in duration to the second sub-periodTb. As described above, not only can the apportionment of time among thefirst sub-period Ta and the second sub-period Tb be adjusted, but it isalso possible to control other parameters such as the range of thedriving voltage (or the driving signal) generated by the signal driver301 in the first sub-period Ta and/or the second sub-period Tb and thehigh voltage applied to the fluorescent materials. By changing thevariation range of the level of the driving signal generated by thegray-scale correction circuit 314 or changing the correctioncharacteristic of the gray-scale correction circuit 314 in conformitywith these adjustment and control, a more desirable picture-drawingoperation can be carried out. It is to be noted that the ratio of thelength of the sub-period Ta to the length the sub-period Tb can beswitched from one value to another on a frame boundary or a lineboundary.

[0048] In addition, the figures do not show the fact that, when the usersets the brightness and the contrast by using a remote controller, notonly can the video signal level be corrected, but it is also possible tocontrol other parameters such as the range of the driving voltage (orthe driving signal) generated by the signal driver 301 in the first-halfsub-period Ta and/or the second-half sub-period Tb and the high voltageapplied to the fluorescent materials so as to allow a betterpicture-drawing operation to be carried out.

[0049] On the basis of synchronization signals such as the horizontaland vertical synchronization signals, the Tb/Ta control circuit 318 setsfor example two horizontal scanning periods as a select period andfurther generates control signals showing the select period's firstsub-period Ta and second sub-period Tb. There are two types of controlsignal. The control signal of the first type is provided forodd-numbered rows and the control signal of the second type is providedfor even-numbered rows. A control signal provided for an even-numberedrow has a waveform lagging behind the waveform of a control signalprovided for an odd-numbered row by about a horizontal scanning period.These control signals control the switches 316 and 317 so that, forexample, in the period t1 to t2, the second driving signal for the firstrow is supplied to the signal driver 320 after being delayed by aboutthe first sub-period Ta and the second driving signal for the second rowis supplied to the signal driver 330 without being delayed. Thesedriving signals are each split into signals to be supplied to theirrespective columns by the data distribution circuit 321. Subsequently,in the period t2 to t3, the split signals are temporarily stored intheir respective latches 322 before being converted by the D/Aconversion circuits 323 into analog driving voltages which are thenapplied to their respective signal electrodes DO1, DO2 and so on.

[0050] In the period t2 to t3, the switches 316 and 317 each select asignal opposite to that shown in the figure. The first driving signalfor the third row is supplied to the signal driver 320 without beingdelayed and the second driving signal for the second row is supplied tothe signal driver 330 after being delayed by about the first sub-periodTa. These driving signals are each split into signals to be supplied totheir respective columns by the data distribution circuit 321.Subsequently, in the period t3 to t4the split signals are temporarilystored in their respective latches 322 before being converted by the D/Aconversion circuits-323 into analog driving voltages which are thenapplied to their respective signal electrodes DO1, DO2 and so on.Thereafter, these select operations are repeatedly carried out in thesame way.

[0051] If the signal electrodes on the odd-numbered rows shown in FIG. 1are pulled up and connected to the signal driver 320 and the signalelectrodes on the even-numbered rows shown in the same figure are pulleddown and connected to the signal driver 330, the conventional signaldriver adopting a simple matrix technique is used as it is. Thus, theembodiment shown in FIG. 1 has an advantage that the present inventioncan be implemented in the conventional signal driver.

[0052]FIG. 5 is a block diagram showing a second embodiment implementingthe display apparatus provided by the present invention. To put it moreconcretely, the block diagram shows the signal driver 301 shown in FIG.1 and a typical signal-processing system for generating a signal appliedto the signal driver 301. In the display apparatus shown in FIG. 5, thesignal driver 301 shown in FIG. 1 comprises a group of odd-numberedsignal electrodes DO1, DO2 and so on, a group of even-numbered signalelectrodes DE1, DE2 and so on, a signal driver 340 for driving theodd-numbered signal electrodes DO1, DO2 and so on and a signal driver350 for driving the odd-numbered signal electrodes DE1, DE2 and so on.The signal drivers 340 and 350 have the same configuration, which isidentical with those of the signal drivers 320 and 330 shown in FIG. 4except that a Ta/Tb signal converter 324 functioning as a changeoverdevice is inserted immediately before each of the D/A conversioncircuits 323.

[0053] Like the gray-scale correction circuit 314 shown in FIG. 4, agray-scale correction circuit 314, which functions as a driving signalgenerator, has a function for transforming the bit count of a digitalvideo signal into a signal with a bit count equal to the input bit countof the signal drivers 340 and 350. However, the input bit count obtainedas a result of the transformation is different from that of theembodiment shown in FIG. 4. In the case of the embodiment shown in FIG.5, a digital video signal with a bit count of 8 is transformed into asignal with a bit count of 9. This signal is used as a driving signalcommon to the sub-periods Ta and Tb in order to generate typicallygray-scale values in the range of 0 to 511 corresponding to the 9 bits.It is the Ta/Tb signal converter 324 that generates a signal for theTa/Tb periods.

[0054]FIG. 6 is a block diagram showing an embodiment implementing theTa/Tb signal converter 324 and FIG. 7 shows a truth table showingtypical operations of the Ta/Tb signal converter 324. If the drivingsignal represents a gray-scale value of n where n is a number in therange of 0 to 255, that is, if the most significant bit b8 of thedriving signal is 0, bits b0 to b7 are output as they are during thesub-period Ta and “0” is output in the sub-period Tb. If the drivingsignal represents a gray-scale value of n where n is a number in therange of 256 to 511, that is, if the most significant bit b8 of thedriving signal is 1, on the other hand, bits b0 to b7 are output as theyare during the sub-period Tb and “1” is output in the sub-period Ta.That is to say, in this embodiment, the most significant bit of the9-bit driving signal obtained as a result of the transformation carriedout by the gray-scale correction circuit 314 is detected and its valueis used as a criterion for determining first and second driving signalsto be apportioned to the first sub-period Ta and the second sub-periodTb. In this case, however, the driving signal representing a gray-scalevalue of 255 causes the signal driver to output the same waveform as thedriving signal representing a gray-scale value of 256. For this reason,considering the fact that the corrected output for a gray-scale value of255 results in the same gray-scale level as the corrected output for thegray-scale value of 256, the gray-scale correction circuit 314 adopts atechnique such as a method of setting the LUT data so as not to use thecorrected output for the gray-scale value of 255 or 256.

[0055] In FIG. 5, the signal driver 340 is a driver in charge of drivinga group of odd-numbered signal electrodes connected to pixels connectedto a group of odd-numbered scanning electrodes. On the other hand thesignal driver 350 is a driver in charge of driving a group ofeven-numbered signal electrodes connected to pixels connected to a groupof even-numbered scanning electrodes. For this reason, the Tb/Ta controlcircuit 318 outputs control signals for controlling the drivers 340 and350 with the control signals'waveforms shifted from each other by halfthe select period of one row. In the embodiment described earlier, thelength of half the select period of one row is equal to the length ofone horizontal period of the video signal.

[0056] In the system shown in FIG. 5, it is necessary to add a dedicatedhorizontal driver including the Ta/Tb signal conversion circuit to thatshown in FIG. 4. However, the logic circuit of the Ta/Tb signalconversion circuit can be realized relatively with ease. In addition,since the line memory 315 is not required, the scale of the circuit canbe suppressed to a relatively small one. Thus, the system shown in FIG.5 has merits such as a cost advantage over that shown in FIG. 4.

[0057]FIG. 8 is a block diagram showing a second embodiment of a pixellayout and electrode wiring of the display apparatus provided by thepresent invention and FIG. 9 is a diagram showing the waveforms ofelectrode select and driving signals generated by electrodes in thesecond embodiment. In the embodiment shown in FIG. 1, scanningelectrodes are laid out, each forming a row. In the case of theembodiment shown in FIG. 8, on the other hand, two rows are driven atthe same time by the same scanning electrode. Thus, the number ofscanning electrodes can be reduced and the fabrication yield can hencebe increased. The number of outputs on a scanning electrode driver 202is only half the number of outputs on the scanning electrode driver 201.Comparison of the typical driving waveforms shown in FIG. 9 with thetypical driving waveforms shown in FIG. 2 indicates that the waveformsof signals generated by the odd-numbered signal electrodes DO1 and DO2connected to pixels on odd-numbered rows in this embodiment are eachdelayed by one horizontal scanning period. In order to generate a delaysignal for such delaying, it is necessary to provide thesignal-processing circuit with a circuit equivalent to the line memory.

[0058]FIG. 10 is a diagram showing an electrode pattern for the secondembodiment shown in FIG. 8 and FIG. 11 is a diagram showing aperspective view of a rear substrate including spacers. The rearsubstrate comprises a glass substrate 421, scanning electrodes 422,signal electrodes 423 and electron emission devices 424.

[0059] In order to build an FED unit, it is necessary to provide a frontsubstrate facing the rear substrate. On the front substrate, which isnot shown in FIG. 11, a fluorescent material and anodes are created. Inorder to realize an even and uniform picture display, spacers 410 eachhaving a typical height of 2 mm may be created between the rearsubstrate and the front substrate so as to keep a uniform gap betweenthe substrates. The spacers 410 are each placed at a location avoidingpixels so as not to obstruct paths of electrons emanating from the rearsubstrate. For a pixel gap of 0.3 mm, each of the spacers 410 isprovided to have a thickness in a range of about 0.05 to 0.1 mm and aheight of approximately 2 mm. In order to erect such thin and tallspacers 410 vertically, support bodies 411 for holding the spacers 410are provided in advance. The support bodies 411 each have a thicknessabout equal to or smaller than the thickness of a spacer 410. It isconvenient to assemble the spacers 410 and the support bodies 411 so asto create a box-like configuration.

[0060] However, some electrons may hit a spacer 410, causing electriccharge to be accumulated on the spacer 410. In order to get rid of thiselectric charge, little conductivity is provided on the surface of everyspacer 410 and the spacers 410 are each put on a scanning electrode 422.In accordance with the present invention, in order to allow two rowseach comprising a group of pixels to be selected by a scanning electrode422, the width of every scanning electrode 422 is made large incomparison with that shown in FIG. 1. Thus, a spacer 410 erected on ascanning electrode 422 is allowed to have a large thickness. For thisreason, the strength of every spacer 410 can be assured. In addition, itis possible to have a margin of tolerance in the alignment precisionbetween a scanning electrode 422 and a spacer 410 erected thereon.

[0061] In assembling the FED unit, a force is applied to the back-faceand front substrates to bind the substrates together. As a result, thespacers 410 provided between the back-face and front substrates getslightly into their respective underlying scanning electrodes 422. Forthis reason, every scanning electrode 422 is created with a relativelylarge thickness so as to make the scanning electrode 422 capable ofplaying the role of a cushion. Thus, when the FED unit is assembled, inorder to prevent the wiring pattern from being injured, the supportbodies 411 are attached to the spacers 410, being floated above thebottoms of the spacers 410 at an altitude at least equal to thethickness of every scanning electrode 422. In addition, the upper sideof each support body 411, that is the side facing the front substrate,is placed at a position lower than the tops of the spacers 410 in orderto avoid effects of electric-charge accumulation. In general, the FEDunit is capable of generating a full-color display by arranging red,green and blue pixels as stripes oriented in the screen verticaldirection. In consequence, the gap between two adjacent pixels in thehorizontal direction is apt to become small but the gap between twoadjacent pixels in the vertical direction is apt to become large. Thus,an electron emitted from an electron emission device 424 is affected byelectric charge accumulated in a spacer 410 or the like existing betweenpixels arranged in the horizontal direction. As a result, it is quitewithin the bounds of possibility that the electron does not properlyarrive at the fluorescent material. For this reason, the thickness ofevery support member 411 placed between pixels arranged in thehorizontal direction had better be made smaller than that of the spacer410 in consideration of the fact that the gap between two adjacentpixels in the horizontal direction is small.

[0062]FIG. 12 is a block diagram showing a third embodiment of a pixellayout and electrode wiring of the display apparatus provided by thepresent invention. The third embodiment is different from the embodimentshown in FIG. 8 in that, in the case of the third embodiment, the groupof pixels on any even-numbered row is shifted in the right direction byhalf the gap between two adjacent pixels away from the group of pixelson any odd-numbered row. In addition, the signal electrode for everyodd-numbered row is pulled up and connected to a signal driver 302 onthe upper side while the signal electrode for every even-numbered row ispulled down and connected to a signal driver 303 on the lower side.

[0063] By shifting pixels on any even-numbered row away from pixels onany odd-numbered row, the number of pixels arranged in the horizontaldirection appears larger, giving rise to an advantage of an improvedresolution sense in the horizontal direction. In addition, by supplyingsignals to the signal electrodes from the upper and lower sides, it ispossible to secure a large value of the connection pitch between thesignal electrodes and the signal drivers.

[0064]FIG. 13 is a block diagram showing a fourth embodiment of a pixellayout and electrode wiring of a display apparatus provided by thepresent invention and FIG. 14 is a diagram showing the waveforms ofelectrode select and driving signals for the fourth embodiment shown inFIG. 13. This embodiment implements a display apparatus having aconfiguration in which the screen is divided into an upper-side area anda lower-side area, which are each driven independently. The number ofoutputs from a scanning electrode driver 203 is equal to the number ofpixels arranged in the vertical direction of the display apparatus. Thescanning electrode SU1 is driven by a signal having the same waveform asa signal for driving the scanning electrode S.D. Likewise, the scanningelectrode SU is driven by a signal having the same waveform as a signalfor driving the scanning electrode S.D. Pixels P11, P12 and so onconnected to the upper-side scanning electrodes SU1 and SU2 areconnected to upper-side signal electrodes DU1, DU2 and so on, which aredriven by an upper-side signal driver 304. Likewise, pixels P31, P32 andso on connected to the lower-side scanning electrodes SD and Sd2 areconnected to lower-side signal electrodes DD1, DD2 and so on, which aredriven by a lower-side signal driver 305. The embodiment shown in FIG.13 can be regarded as an equivalent to the embodiment shown in FIG. 8wherein the group of pixels on each odd-numbered electrode is allocatedto the upper-side area whereas the group of pixels on each odd-numberedelectrode is allocated to the lower-side area. Since the embodimentshown in FIG. 13 has the same operations as the embodiment shown in FIG.8, detailed explanation is omitted.

[0065] In spite of the fact that the embodiment shown in FIG. 13requires a frame memory for signal processing, this embodiment has anadvantage that the number of signal electrode wires can be reduced tohalf the number of signal electrode wires in the embodiment shown in.FIG. 1. In the typical driving waveforms shown in FIG. 14, the timing todrive the bottom group of pixels in the upper-side area is shifted fromthe timing to drive the top group of pixels in the lower-side area.Thus, a shift in moving-picture display timing may result in, givingrise to a phenomenon in which, for example, a vertical line moving inthe horizontal direction appears as a vertical line broken at itscenter. This problematic phenomenon can be eliminated by adjusting thetimings, with which the bottom group of pixels in the upper-side areaand the top group of pixels in the lower-side area are driven. That isto say, this problem can be solved by substantially reversing thescanning directions of the upper-side and lower-side areas.

[0066] In the present invention's embodiments described above, astypical electron emission devices of the FED unit, electron emissiondevices of the MIM type are employed. However, it is also possible toemploy electron emission devices of a variety of other types such as aSpindt type, a surface conduction type and a carbon nano tube type. Inthe embodiments described above, an FED unit is used as the displayapparatus, but the scope of this present invention is not limited to theFED unit. That is to say, the present invention can also be applied to adisplay apparatus employing an ELD (Electro-Luminescent Display) units,an OLED (Organic Light-Emitting Diodes) or other devices. To put it indetail, the present invention can also be applied to a display apparatusincluding electron injection devices for injecting electrons (or holes)to a light emission layer and the light emission layer for emittinglight due to radiation of the electrons (or holes) injected by theelectron injection devices to the light emission layer, wherein thenumber of electrons (or holes) injected by the electron injectiondevices can be controlled by adjusting a select voltage applied to ascanning electrode connected to the electron injection devices and adriving voltage connected to a signal electrode connected to theelectron injection devices.

[0067] As described above, in accordance with the present invention, itis possible to improve the gray-scale performance and, hence, display apicture with a high intensity and a high resolution. Thus, in a flatdisplay apparatus such as a FED unit, a picture having a high qualitycan be displayed.

What is claimed is:
 1. A display apparatus comprising: a front substrateon which a fluorescent material is provided; a rear substrate disposedopposite to said front substrate and having a plurality of electronemission devices laid out thereon to form a matrix, each of saidelectron emission devices radiating electrons to said fluorescentmaterial; and a driver capable of applying two or more driving voltagessequentially, which are generated on the basis of an input video signaland have levels independent from each other, during a select period toat least one row of specific electron emission devices selected amongsaid electron emission devices.
 2. A display apparatus according toclaim 1 wherein: said input video signal is a digital video signal; andsaid two or more driving voltages are generated on the basis of adigital signal obtained as a result of converting the bit count of saiddigital video signal.
 3. A display apparatus comprising: a rearsubstrate including: a plurality of scanning electrodes extended in ascreen horizontal direction; a plurality of signal electrodes extendedin a screen vertical direction; and a plurality of electron emissiondevices placed at intersecting points of said scanning electrodes andsaid the signal electrodes, each of electron emission devices emittingelectrons; a front substrate disposed opposite to said rear substrateand provided with a fluorescent material emitting light due to electronsradiated thereto by said electron emission devices; a scanning driverfor applying to said scanning electrodes a select voltage for selectingat least one row of specific electron emission devices selected amongsaid electron emission devices during a predetermined select period; anda signal driver for applying to said signal electrodes a driving voltagehaving a level depending on an input video signal for driving saidelectron emission devices; wherein: the duration of said select periodis determined by the output period of said select voltage; said selectperiod is divided into a plurality of sub-periods; and said drivingvoltage is applied in each of said sub-periods.
 4. A display apparatusaccording to claim 3 wherein the level of said driving voltage appliedto said signal electrodes is changed for each of said sub-periods.
 5. Adisplay apparatus comprising: a plurality of scanning electrodesextended in a screen horizontal direction; a plurality of signalelectrodes extended in a screen vertical direction; a screen on which aplurality of display devices are placed at intersecting points of saidscanning electrodes and said signal electrodes to form a matrix; ascanning driver for applying to said scanning electrodes a selectvoltage for selecting at least one row of specific display devicesselected among said display devices during a predetermined selectperiod; and a driving signal generator capable of generating first andsecond driving signals, which have values independent from each otherand each serve as a signal for driving said display devices, on thebasis of an input video signal; wherein: the duration of said selectperiod of said row of specific display devices is determined by saidselect voltage generated by said scanning driver; and in said selectperiod, driving voltages obtained on the basis of said first and seconddriving signals generated by said driving signal generator are appliedconsecutively to said signal electrodes.
 6. A display apparatuscomprising: a plurality of scanning electrodes extended in a screenhorizontal direction; a plurality of signal electrodes extended in ascreen vertical direction; a screen on which a plurality of displaydevices are placed at intersecting points of said scanning electrodesand said signal electrodes to form a matrix; a scanning driver forapplying to said scanning electrodes a select voltage for selecting atleast one row of specific display devices selected among said displaydevices during a predetermined select period; a driving signal generatorcapable of generating first and second driving signals, which havevalues independent from each other and each serve as a signal fordriving said display devices, by conversion of the bit count of an inputdigital video signal; a switch for outputting said first driving signalgenerated by said driving signal generator during a first period for theselect period determined by an output period of the select voltagegenerated by said scanning driver and outputting said second drivingsignal generated by said driving signal generator during a second periodfor the select period determined by an output period of the selectvoltage generated by said scanning driver; and a D/A converter forconverting said first and second driving signals output by said switchinto analog signals and for applying the analog signals to said signalelectrodes as first and second driving voltages respectively.
 7. Adisplay apparatus according to claim 6 wherein: said display deviceincludes an electron injection device for injecting electrons and alight emission layer for emitting light due to electrons (or holes)radiated thereto from said electron injection device; and the number ofelectrons (or holes) radiated by said electron injection device iscontrolled by the select voltage applied to said scanning electrodeconnected to said display device and the driving voltage applied to saidsignal electrode connected to said display device.
 8. A displayapparatus according to claim 6 wherein the duration of said firstsub-period is made different from the duration of said secondsub-period.
 9. A display apparatus according to claim 6 wherein: theduration of said first sub-period is made shorter than the duration ofsaid second sub-period; in an operation to produce a dark gray-scaledisplay, gray-scale control is executed so as to set said second drivingvoltage applied in said second sub-period at a fixed level of no lightemission and vary said first driving voltage applied in said firstsub-period; and in an operation to produce a bright gray-scale display,gray-scale control is executed so as to set said first driving voltageapplied in said first sub-period at a fixed level of a substantiallymaximum light emission and vary said second driving voltage applied insaid second sub-period.
 10. A display apparatus according to claim 6,further comprising an extraction circuit for extracting characteristicsof said input video signal, wherein the durations of said first andsecond sub-periods or ranges of said driving voltages applied in saidsub-periods are changed in accordance with characteristic extractionresults output by said extraction circuit.
 11. A display apparatusaccording to claim 6, further comprising a brightness or contrastsetting unit, wherein the durations of said first and second sub-periodsor ranges of said driving voltages applied in said sub-periods arechanged in accordance with a brightness or contrast set value.
 12. Adisplay apparatus according to claim 6 wherein said driving signalgenerator is a gray-scale correction circuit having a function tocorrect a gray-scale characteristic's discontinuity caused by acombination of said first and second driving voltages applied in saidfirst and second sub-periods respectively.
 13. A display apparatusaccording to claim 6 wherein said driving signal generator generatessaid first and second driving signals by converting said digital videosignal into a signal having a bit count greater than the bit count ofsaid digital video signal.
 14. A display apparatus according to claim 6wherein the sum of the bit counts of said first and second drivingsignals generated by said driving signal generator is greater than thebit count of said digital video signal.
 15. A display apparatusaccording to claim 6 wherein the bit counts of said first and seconddriving signals generated by said driving signal generator are eachequal to the bit count of a digital signal that can be handled by saidD/A converter.
 16. A display apparatus according to claim 6 wherein saidscanning driver outputs a select voltage for selecting two rows of saiddisplay devices at one time in a sequential scanning operation carriedforward in said screen vertical direction.
 17. A display apparatusaccording to claim 6 wherein said scanning driver outputs a selectvoltage for selecting two rows of said display devices at one time in asequential scanning operation carried forward in said screen verticaldirection in such a way that a select period of one of said two selectedrows does not completely coincide with a select period of the otherselected row.
 18. A display apparatus according to claim 6 wherein saidscanning driver outputs a select voltage for selecting at least one rowof said display devices located on the upper half side of said screenand at least one row of said display devices located on the lower halfside of said screen.
 19. A signal driver employed in a displayapparatus, said apparatus having a plurality of scanning electrodesextended in a screen horizontal direction; a plurality of signalelectrodes extended in a screen vertical direction; a plurality ofdisplay devices placed at intersecting points of said scanningelectrodes and said the signal electrodes; and a screen comprising saiddisplay devices laid out thereon to form a matrix; wherein said signaldriver used for applying a driving voltage for driving said displaydevices to said signal electrode, said signal driver comprising: ann-bit gray-scale signal input terminal for inputting an n-bit gray-scalesignal where n≧8; a sub-period select signal input terminal forinputting a sub-period specification signal for specifying one of msub-periods obtained as a result of dividing a select period of saidscanning electrodes where m≧2; an output circuit for outputting kvoltage levels where k≦(the nth power of 2)/m; and a signal converterfor selecting one of said k voltage (or current) levels on the basis ofsaid n-bit gray-scale signal and said sub-period specification signal.20. A display apparatus comprising: a rear substrate including: aplurality of scanning electrodes extended in a screen horizontaldirection; a plurality of signal electrodes extended in a screenvertical direction; a plurality of electron emission devices placed atintersecting points of said scanning electrodes and said the signalelectrodes, each of said electron emission devices emitting electrons; afront substrate disposed opposite to said rear substrate and providedwith a fluorescent material emitting light due to electrons radiatedthereto by said electron emission devices; and spacers placed betweensaid rear substrate and said front substrate to create a space betweensaid rear substrate and said front substrate; wherein: each specific oneof said scanning electrodes is connected to two rows each comprising agroup of specific electron emission devices; said two rows eachcomprising a group of specific electron emission devices are connectedrespectively to two different ones of said signal electrodes; and eachof said spacers is located substantially at the center of said two rowseach comprising a group of specific electron emission devices on saidspecific scanning electrode.
 21. A display apparatus according to claim20 wherein: each two specific ones of said spacers are erected ondifferent ones of said scanning electrodes to create a box-likeconfiguration in conjunction with support members allowing said twospecific spacers to support each other; and the upper sides of saidsupport members are placed at positions lower than the tops of saidspecific spacers and the lower sides of said support members are floatedabove the bottoms of the spacers at an altitude at least equal to thethickness of each of said scanning electrodes.